Hydrocracking processes can be used to upgrade higher boiling materials, such as residuum, typically present in heavy crude oil, by converting it into more valuable lower boiling materials. For example, at least a portion of the residuum feed to a hydrocracking reactor may be converted to a hydrocracking reaction product. The unreacted residuum may be recovered from the hydrocracking process and either removed or recycled back to the hydrocracking reactor in order to increase the overall residuum conversion.
The residuum conversion in a hydrocracking reactor can depend on a variety of factors, including feedstock composition; the type of reactor used; the reaction severity, including temperature and pressure conditions; reactor space velocity; and catalyst type and performance. In particular, the reaction severity may be used to increase the conversion. However, as the reaction severity increases, side reactions may occur inside the hydrocracking reactor to produce various byproducts in the form of coke precursors, sediments, other deposits, as well as byproducts, such as asphaltenic pitch.
One type of cracking reactor used in various processes includes an ebullated bed hydrocracking reactor. The addition of an intermediate or pre-deasphalting step to an ebullated bed process may enhance the performance of the ebullated bed process, such as by increased conversion and production of residual oils with high stability. However, there is inevitably a volume of pitch that needs to be disposed of. The pitch, if it is to be converted to fuel oil, requires a very high volume of a lighter cutter stock, such as light cycle oil from an FCC unit. The pitch can alternatively be fed to a Delayed Coking unit, but pitch is not a very desirable feedstock. The pitch could also be converted to asphalt, but demand can be seasonal and the quality of pitch may not meet local specifications. The pitch could be gasified, but costs are generally prohibitive. Pitch is therefore not a desired byproduct.